In high-speed communication systems, there exists a problem of phase ambiguity during phase modulation. Current solutions include the following two methods. One is a training sequence method applied to an absolute phase detection system, that is, a phase ambiguity angle is detected through the training sequence of a known phase, and then a proximity signal is corrected accordingly. Such method can well solve the problem of phase ambiguity, but increases the system overhead. The other method employs a phase differential modulation to correct the problem of phase ambiguity, but a performance is reduced compared with training sequence method.
In four-dimensional modulation format, such as Set-partitioning Quadrature Amplitude Modulation (SP-QAM), divided based on Polarization Multiplexed M-state Quadrature Amplitude Modulation (PM-MQAM), the QAM is a quadrature amplitude modulation such as 32-SP-QAM and 128-SP-QAM. In such pattern, N bits which make up the signal are divided into two subsets. One subset makes up the SP-QAM signal n, and the other is a checking bit. In this way, each symbol of the SP-QAM signal carries n bits valid information, a minimum Euclid distance is increased compared to PM-MQAM. Taking 128-SP-QAM for example, there are 8 bits in total including 7 bits information. As shown in FIG. 1, d1 to d7 are the valid information. FIG. 2, FIG. 3 and FIG. 4 show that this method achieves an association of an X polarization state and a Y polarization state. For example, when the valid information is 1101 011, and the last bit can only be 0 or 1 according to odd parity or even parity. For example, the last bit can be 1 if the even parity is employed.
A decision algorithm at a reception terminal generally includes the following steps.
In step 1, symbols on the Y polarization state and the X polarization state are decided, bit information is obtained by mapping, and the bit information is checked and analyzed. Data that meets the check is reserved.
In step 2, symbols X and Y which fail to meet the check are re-decided in a next-nearest decision domain, meanwhile, a distance between an original symbol and the decision point is recorded, and data with shorter renewal and shorter distance of X and Y are compared.
In step 3, error is compared.
Before the error comparison, it is necessary to carry out the phase ambiguity problem. If the method based on training sequence is completed before the check decoding in the SP-QAM system, a parity misjudgment caused by phase ambiguity can be avoided. However, a disadvantage of this method is the training sequence increases the system overhead.
For SP-QAM system using differential phase, if the differential decoding is completed before the parity correction of the SP-QAM, the parity misjudgment caused by phase ambiguity in the system can also be avoided. However, the system performance is reduced. if the differential decoding is after the parity correction, the parity misjudgment caused by the phase ambiguity leads to large consecutive errors.